Distortion

Typical for extreme range zoom lenses the 18-200mm OSS exhibits a fairly hefty amount of barrel distortion at 18mm (~4.5%). However, beyond the wide end the problem is actually very well controlled with non-existent distortion at 24mm and slight pincushion distortion from 50mm onwards.

Move the mouse cursor over the focal length text marks below to observe the respective distortion

Vignetting

The Sony lens has a very moderate vignetting characteristic even at max. aperture at the extreme ends of the zoom range. There's only a slight light fall-off of around 0.6EV here which is a nothingness compared to other lenses in this class. If you stop down just a little bit the amount of vignetting is absolutely negligible. This may be the effect of an auto-compensation algorithm in the camera (RAWs behave the same).

MTF (resolution)

Our test sample had a bit of tilted focus plane but the produced resolution figures were pretty impressive for such a lens nonetheless. In the wide portion of the zoom range the lens is capable of producing good to very good quality results across the frame. The quality in the image center is even on an excellent level. There's a bit of a performance drop at 100mm. The center resolution remains very fine here but the borders are quite soft at f/5.6 and it makes sense to stop down to f/8 or f/11 for an extra kick in quality. The border situation improves a little at 200mm @ f/6.3 but the center quality is further reduced and the contrast level is somewhat lower. The quality is still fine here but it is no longer "bitingly sharp".

Below is a simplified summary of the formal findings. The chart shows line widths
per picture height (LW/PH) which can be taken as a measure for sharpness.
If you want to know more about the MTF50 figures you may check out the corresponding
Imatest Explanations

Chromatic Aberrations (CAs)

The Sony 18-200mm f/3.5-6.3 OSS struggles a bit regarding the amount of lateral CAs (color shadows at harsh contrast transitions). The problem is very pronounced at 18mm and 200mm with a average CA pixel width of around 2px at the image borders. This is easily visible in images unless you correct the problem (e.g. via Photoshop). The subjective quality perception is also affected by this. The issue isn't quite as extreme in the middle part of the zoom range.